Hello.
Im a beginner in blender so probably a simple fix for this.
When I apply a Boolean to the below object I get extra lines appear on the object.
Like so.
What can I do to stop this happening?
Thanks.
Welcome 
…
this is normal because blender (and also some other model software) can’t make polygons with holes… so there are at leat two… but doesn’t matter if you are not using any smooth shading because this polygons are coplanar…
( bevels are another story…)
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They are called connecting lines because, unlike cad programs, Blender needs to know how the holes are connected to the outer edges. Depending on the angle of these, as you bevel, you can get strange overlaps. Personally, I would do a knife which passes through the centre of the long edge of the hole and connects to the outer edge, as indicated by the red lines, on both sides then select the slanted edges and using ctrl+x dissolve edges, to leave a tidier result.
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